The present invention relates to an apparatus and a method for simulating a signal composed of plurality of individual signals, as it can be used in particular for realistic simulation of a frequency spectrum.
Methods for localization of transmitters or emitters of radio waves, respectively, such as radio direction finding, are methods where an individual position of a radio direction finder or a position and/or direction of a transmitter or emitter, respectively, can be determined, for example by direction finding or timing of a radio signal. Frequently, a relative velocity between transmitter and receiver can also be determined. Radio direction finders generally behave purely passive and select only electromagnetic waves that are radiated at other locations by other devices. Generally, for radio direction finding, one direction finding receiver and one or several antennas are necessitated.
For testing receivers or radio direction finders, respectively, that are used, for example, in radio detection or radio direction finding, respectively, in a realistic scenario, it is necessitated to generate a plurality of modulated time-varying signals or emitter signals, respectively, having realistic signal content within a frequency range that is as broad as possible. Further, for testing radio direction finders, it is necessitated to provide this plurality of signals at several outputs of a test device with exactly defined phase, frequency and level differences.
FIG. 7 shows the principle of a so-called interferometer direction finding, wherein phase relationships between several spatially separate similar individual antennas are used for direction finding.
FIG. 7 shows a transmitter or emitter 100, respectively, a plurality of receiving antennas 110-1, 110-2, . . . , 110-N and a combination means 120. The emitter 100 emits a modulated time-varying emitter signal s(t) at a signal location, which is received by the receiving antennas 110-1 to 110-N. Since the receiving antennas 110-1 to 110-N are spatially separate from each other, the respective receiving signals r1(t) to rN(t) have different phase relationships to each other. For further processing or determination of the signal location, respectively, the different receiving signals r1(t) to rN(t) can be combined to an overall signal or combined signal rges(t) by the combination means 120.
The scenario shown in FIG. 7 could also be reversed. Here, the antennas 110-1, 110-2, . . . , 110-N could act, for example, as transmitting antennas at different signal locations and transmit transmission or emitter signals, respectively, s1(t) to sN(t) to a receiver 100, which then receives a signal sges(t) composed of the plurality of individual signals s1(t) to sN(t).
In practical tests of receivers or radio direction finders, respectively, that are to take place in a laboratory, it is advantageous to simulate realistic scenarios with regard to signal and receiver positions, as they are shown schematically in FIG. 7 for an emitter and several receiving antennas. Obviously, a plurality of emitters is also possible. Here, in principle, the necessitated number of emitter signals can be generated by standard measurement technology, which consists of a combination of a so-called arbitrary waveform generator (AWG) with a frequency converter. However, the disadvantage of this method is that respective memory requirements in an AWG are comparatively high, since the emitter signals have to be stored in a transmittable signal bandwidth. In order to be able to simulate a 20 MHz wide spectrum for only 10 seconds, already approximately 1.5 gigabyte of data are necessitated. Further, the time needed for calculating the data is long and is normally by orders of magnitude above the actual duration of the emitter signal. Changing the transmitter or emitter configuration, respectively, in real time by user intervention is also not possible in such a configuration, which also presents a significant disadvantage. For avoiding this disadvantage it would be necessitated to use one AWG with variable clock frequency and frequency converter with variable center frequency per emitter. However, this is normally inefficient.